Currently, an estimated 4.8% of the U.S. population reports past-year nonmedical use of prescription opioids (NMU-POs), an increase of over 40% from 10 years ago. Among young adults 18-25 years old, the prevalence is markedly higher (11.1%). Young NMU-POs are often naive to the serious risks associated with such use, tending to view prescription drugs as relatively safe in comparison to illicit street drugs. Although the majority of NMU-POs begin ingesting POs orally, emerging research demonstrates that a significant minority may transition to heroin use and/or injection drug use (IDU), often within 2-3 years of NMU-PO initiation. This suggests that NMU-PO may function as a new pathway into heroin use and dependence and IDU among youth. IDU and heroin use is associated with significant negative health outcomes, increased sexual risk behavior and increased susceptibility to route-specific infections. These risks are exacerbated by the fact that harm reduction services aimed at preventing HIV among IDUs typically serve older heroin injectors who have been found to differ in age, race/ethnicity and other characteristics from the emerging group of young NMU-POs. The striking increases in NMU-PO throughout the U.S. are thus alarming from a public health perspective, and may lead to increases in HCV and HIV prevalence among groups of NMU-POs. However, research on drug-use trajectories, drug and sexual risk behaviors and, most notably, HIV, HCV and STI prevalence rates, in NMU-PO populations remains scarce. Although very little research to date has examined the sexual risk behaviors of NMU-POs, the body of research on young urban drug users, as well as our team's pilot study of 18-29 year-old NMU-POs, suggests that high levels of sexual risk-taking may be common among NMU-PO young adults. The proposed mixed-methods study will assess the drug and sex-related HIV/HCV/STI risk associated with NMU-PO among young adults. Specifically, we will: characterize the drug-use patterns and sexual behaviors of a representative RDS-recruited sample of 600 NMU-POs (ages 18-29) in the New York City area; determine the proportion of this population that engages in IDU and/or heroin use; and identify individual, network and socio-structural factors that may predispose some NMU-PO young adults to engage in IDU and/or the use of heroin. In addition, key objectives of this research will be to systematically document the HIV/HCV/Chlamydia and Gonorrhea risk and preventive behaviors, knowledge, testing practices and prevalence rates of NMU-PO young adults. Foundational research that elucidates drug- and sex-related risk behaviors and HIV, HCV and other STI prevalence rates among this growing group of young NMU-POs is necessary for the development of disease prevention messages and interventions tailored to their specific characteristics, needs and drug-use and sexual practices. Furthermore, this research will help identify young NMU-POs who are likely to transition to heroin use and/or IDU, enabling interventions designed to prevent or delay these behaviors to be targeted to groups of youth who are most at-risk.